This invention relates to keyboards and more particularly to a type of keyboard particularly suitable for mass production.
A great number of terminals providing data to data processing systems or telephone switching systems require the use of keyboards for introducing the data. Consequently, it becomes necessary to provide the cheapest keyboards which at the same time show a sufficient quality to be reliable in all the operating conditions and give the operator a tactile response to a successful actuation of a key.
There are many keyboards available on the market. In one type, the contacts are ensured through metal diaphragms. In another type, the contacts are ensured through a metal element moving when an associated key is depressed to contact a fixed metal element provided on a printed circuit board. In such keyboards, which are generally thin, no conventional means are provided to give the operator a tactile response to a successful actuation of a key, i.e., to the closing of the associated contact.
The typical tactile response means generally include springs which are too bulky to be embodied into a thin keyboard. Consequently, attempts have been made to replace these means by elements not increasing the keyboard thickness such as the ones described in French Pat. No. 1,590,517.
The keyboard described in the above identified patent consists of a plate of flexible material provided with protrusions serving as push buttons. In a first embodiment, this plate is made from metal and is separated from a fixed printed circuit board provided with a series of metal contacts, by an insulating plate provided with holes aligned with the protrusions. When the operator depresses one of the protrusions, said protrusion is distorted and comes into contact with a contact element of the printed circuit board. When pressure is released, the protrusion comes back to its original shape with a click. Thus the operator feels that the key has been depressed and that the contact has been closed.
The keyboard according to the above-described embodiment is very simple but shows a certain number of disadvantages, namely, the metal plate is to be connected to a power source in order to have some current drawn by the fixed contact elements when the associated keys are depressed. In addition, indicia such as letters, figures or signs are to be engraved in the metal push buttons and this operation is expensive.
These disadvantages are overcome in a second type of keyboard described in the same French patent. This keyboard includes a plate of insulating material provided with protrusions used as push buttons. These protrusions are associated with moving contacts coming into contact with the fixed contact elements of a printed circuit board when pressure is applied to the push buttons.
The moving contacts are associated with the push buttons through coupling elements consisting in a preferred embodiment of small balls located under the protrusions and able to be slightly distorted.
In the two above-described embodiments, the keyboard tactile response consists in the click due to the depression of the protrusions provided in the plate of flexible material. However, in the first embodiment, the tactile response is not the best one since the protrusion has to move against the fixed contact element. In the second embodiment, this disadvantage is overcome due to the flexibility of the coupling elements, but the keyboard is much more difficult to be assembled and since it includes a high number of parts, it would be necessary to adjust the position of the various parts key by key, to ensure a good operation of said keyboard.
In addition, in both embodiments, dust can be introduced between the fixed and moving contacts or the metals can become oxidized, which prevents both elements from ensuring a good contact.
An object of this invention is to provide a keyboard which can be easily assembled without requiring any individual adjustment of the keys.
Another object of this invention is to provide a sealed keyboard in which the contacts are self-cleaned.
Another object of this invention is to provide an inexpensive keyboard with good tactile response and the capability of being shielded.
Another object of this invention is to provide a keyboard of small dimensions.
According to this invention, the keyboard includes a minimum number of parts which can be easily assembled, which allows mass production and consequently low cost.
The keyboard comprises four main elements: the keys arranged as a matrix, an insulating support, conducting contact strips and a plate of transparent insulating material. The insulating support is provided with a number of openings equal to the number of keys in the keyboard. The openings are arranged as a matrix. The individual keys are provided with printed or engraved indications such as letters, figures or signs and are housed in the openings flush with the upper surface of the support. Housings for the conducting contact strips are provided in the lower surface of the support. The strips are divided into two sets. The strips of the first set are flat and parallel and are used as fixed contact elements. The strips of the second set are parallel and perpendicular to the strips of the first set. The strips of the two sets intercross and the number of intersections is equal to the number of keys. The strips of the second set are located in the support housings and their shape is such that they hold the keys in their high position at rest. When a key is depressed by an operator, it comes to bear against the corresponding strip of the second set and this strip is brought into contact with one of the strips of the first set. The strips rub one against the other at the contact point in a wiping action and this cleans said contact point. Thus, the risks of contamination of the metals by dust deposit or oxidation are avoided.
A plate of transparent insulating material covers and is secured to the support upper surface so as to hold the keys in their openings. Cups as portions of a sphere are formed in this plate and lay in correspondence with the keys. When finger pressure is applied to one of these cups to depress the associated key, the cup goes out of shape. The cup comes back into its original shape when pressure is released, and thus a tactile response of the keyboard is obtained.
To ensure a complete tightness of the keyboard, a protective plate can be fixed to the support lower surface.